Method for measuring hydrogen peroxide in water

ABSTRACT

An apparatus and method for measuring hydrogen peroxide concentration in water to an accuracy of 0.1 ppm comprises a colorimetric assay method to determine hydrogen peroxide concentration. The assay is monitored spectophotometrically at a desired wavelength. Each sample is corrected relative to a control sample and hydrogen peroxide concentration determined with respect to a standard curve.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/001,204 filed on May 21, 2014 incorporated herein byreference in its entirety.

FIELD OF INVENTION

The present invention relates to an apparatus and method for measuringhydrogen peroxide to an accuracy of 0.1 ppm in water, particularly indrinking water.

BACKGROUND

Stabilized hydrogen peroxide solutions used for water disinfection, suchas HUWA-SAN™ owned by Roam Chemie NV of Houthalen, Belgium, and SANOSIL™owned by Sanosil Ltd. of Hombrechtikon, Switzerland are known in theart. Such hydrogen peroxide (H₂O₂) solutions are proprietary and arestabilized by silver ions or silver colloid in minute concentrations.Other stabilized hydrogen peroxide solutions are stabilized by alcohols,acids or other compounds. Depending on the solution, the stabilizerprevents the hydrogen peroxide from oxidizing too quickly when itcontacts water, thereby allowing the solution to mix with the waterbefore binding to and disinfecting undesirable microorganisms andchemicals.

Various apparatuses exist to measure the concentration of hydrogenperoxide in water including with chemiluminescent, fluorometric,amperometric and colorimetric sensors. The prior art sensors anddetection systems were built to measure hydrogen peroxide concentrationthresholds in swimming pool water treatment systems where regulationsallow maximum levels not to exceed, for example, 150 mg/L (150 ppm), andtypical operating concentrations are between 50-100 ppm. Otherregulations have standards in the same order of magnitude.

In drinking water regulation, however, the acceptable concentrationthresholds are much lower, often in the order of under 10 ppm. Forexample, in Ontario, Canada, operating concentrations for drinking waterare between 2-8 ppm. Existing detection methods are inadequate toquickly measure the concentration of hydrogen peroxide in water at suchlow levels at an accuracy better than ±3 ppm.

There is a need for a measurement apparatus and method to quickly detectlow concentrations of hydrogen peroxide in water, including drinkingwater, in the order of 10.0 ppm or less and to an accuracy of 0.1 ppm.Such techniques must not be affected by pH, temperature or watercomposition.

SUMMARY OF THE INVENTION

In a first aspect of the invention, an apparatus for measuring hydrogenperoxide levels in water using a colorimetric assay method is provided.The apparatus comprises a measurement cell for containing a watersample; a light transmitter configured to emit light at a selectedwavelength at the measurement cell; a photodiode receiver configured toreceive light passing through the measurement cell and a reagent in areagent vial. The reagent comprises a reagent compound configured toreact with hydrogen peroxide to form a reaction product, the reactionproduct is adapted to absorb light at the selected wavelengthproportional to the amount of hydrogen peroxide in the water sample. Theapparatus also comprises a surfactant and a solvent. Within theapparatus, there is a first network of pipes connecting source water toa buffer jar, the buffer jar to a supply valve, the supply valve to theapparatus measurement cell, and a second network of pipes connecting thereagent vial to a reagent valve, the reagent valve to the measurementcell, and the measurement cell to a drain valve; and a control unit; thecontrol unit is configured to cause a first colorimetric measurement ofa first water sample free of reagent and a second colorimetricmeasurement of a second water sample mixed with reagent. The controlunit determines the difference between the first and second measurementsand compares the difference against a pre-determined standard curve ofdiluted hydrogen peroxide to determine and report the concentration ofhydrogen peroxide in the water sample, accurate to 0.1 mg/L.

In one embodiment the reagent compound is potassium bis (oxalato)oxotitanate (IV) DI.

In another embodiment, the selected wavelength is 470 nm.

In a further embodiment the light transmitter is a LED light emitter.

In another embodiment the reagent comprises potassium bis (oxalato)oxotitanate (IV) DI, EDTA di-sodium salt dihydrate and polyoxyethylene(23) lauryl ether mixed in a solvent.

In one embodiment the solvent is sulfuric acid 99%: p.a. 10% solution.

In a further embodiment the predetermined standard curve comprises datapoints from 0 ppm to 150 ppm.

In another embodiment the predetermined standard curve comprises datapoints from 0 ppm to 100 ppm.

In another aspect of the invention, there is provided a method ofmeasuring hydrogen peroxide levels in water using a colorimetric assaycomprising transferring a first water sample to a measuring cell;determining a first absorbance measurement of light at a selectedwavelength as a null measurement; removing the first sample from themeasurement cell; transferring an aliquot of a reagent consisting of areagent compound configured to react with hydrogen peroxide to form areaction product, the reaction product adapted to absorb light at theselected wavelength proportional to the amount of hydrogen peroxide inthe water sample to the measurement cell; filling the measurement cellwith a second water sample; determining a second absorbance measurementof light at the selected wavelength as a test measurement; emptying themeasurement cell and rinsing with sample water; and a control unitadapted to determine the difference between the first and secondmeasurements and compare the difference against a pre-determinedstandard curve of diluted hydrogen peroxide to determine and report theconcentration of hydrogen peroxide in the water sample to an accuracy of0.1 mg/L.

In another embodiment the method uses the reagent compound potassium bis(oxalato) oxotitanate (IV) DI.

A further embodiment of the method relates to the selected wavelength is470 nm.

Another embodiment of the invention provides the reagent comprisingpotassium bis (oxalato) oxotitanate (IV) DI, EDTA di-sodium saltdihydrate and polyoxyethylene (23) lauryl ether mixed in a solvent.

In another embodiment the solvent is sulfuric acid 99%: p.a. 10%solution.

In one embodiment the predetermined standard curve comprises data pointsfrom 0 ppm to 150 ppm.

In a further embodiment the predetermined standard curve comprises datapoints from 0 ppm to 100 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and not limitation in thefollowing figures, in which like references indicate similar elements.

FIG. 1 is a box diagram of one embodiment of an apparatus of the presentinvention.

FIG. 2 is a diagram of one embodiment of FIG. 1 showing the analysisunit and control unit.

FIG. 3 is a close-up diagram of FIG. 2 including an expanded portionillustrating the buffer jar.

FIG. 4 is a schematic of a close-up of the apparatus of FIG. 2 showingthe buffer jar, reagent vial and measurement cell.

FIG. 5 is a schematic of a close-up of the apparatus of FIG. 2 showingthe direction of flow and measurement cell.

FIG. 6 is a schematic of a close-up of the apparatus of FIG. 1 showingthe measuring cell, LED and receiver.

FIG. 7 is a chart of measured hydrogen peroxide concentration used tocalibrate one embodiment of the apparatus of the present invention.

FIG. 8 is a logarithmic chart of hydrogen peroxide values generated tocalibrate one embodiment of the apparatus of the present invention.

FIG. 9 is a pre-determined standard curve generated by plottingAbsorbance data relative to concentration of hydrogen peroxide for usein determining the concentration of hydrogen peroxide in a measured testsample, in accordance with one embodiment of the present invention.

FIG. 10 is a pre-determined standard curve generated by plottingdigitized measurement data relative to concentration of hydrogenperoxide for use in determining the concentration of hydrogen peroxidein a measured test sample in accordance with one embodiment of thepresent invention.

DETAILED DESCRIPTION

Example embodiments, as described below, may be used to provide anapparatus and method to quickly measure hydrogen peroxide in water atvery low concentrations.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this invention belongs. Although methods and materialssimilar or equivalent to those described herein can be used in thepractice or testing of the present invention, suitable methods andmaterials are described below. In the case of conflict, the presentspecification, including definitions, will control. In addition, thematerials, methods, and examples are illustrative only and not intendedto be limiting.

The present invention is a colorimetric assay method to determinehydrogen peroxide concentration. A colorimetric method is based onproduction of a reaction product that absorbs light at a selectedwavelength. Preferably a reagent compound used to produce the reactionproduct does not have significant light absorption properties at theselected wavelength. Formation of the reaction product is proportionalto the amount of hydrogen peroxide in the water sample. Quantificationof the reaction product is measured and converted to a H₂O₂concentration based on a standard curve.

A preferred colorimetric method is based on production of a reactionproduct that produces a yellow to orange coloured complex when potassiumbis(oxalato)-oxotitanate (IV) reacts with hydrogen peroxide to form areaction product adapted to absorb light at 470 nm proportional to theamount of hydrogen peroxide in the sample. Quantification of thereaction product is measured at 470 nm and converted to a H₂O₂concentration based on a calibration curve. The photodiode measurementdata produced by the reaction product of the above reactant with H₂O₂has been determined to correlate logarithmically with H₂O₂concentration. Alternatively, other wavelengths may be used such as 400nm. The wavelength that gives the maximum absorbance of a colouredreaction product is one consideration in choosing a selected wavelength.Additionally the resulting standard curve and degree of linearity thatcan be achieved may vary at each wavelength. In one embodiment of thepresent invention, the wavelength is selected to be 470 nm. The standardcurve generated with this data produces a close to linear standard curveand a high degree of accuracy is thereby achieved. Other wavelengths oflight are contemplated.

One objective is to obtain the greatest accuracy by facilitating theclosest possible adherence to the Lambert Beer principles of lightabsorption between the transmitter and receiver, in accordance with theformula:

$\frac{I_{1}}{I_{0}} = {10^{- E} = 10^{- {eed}}}$

In one embodiment, an apparatus 100 is provided in FIG. 1. The maincomponents of the apparatus 100 include control unit 35, measurementcell 15, buffer jar 10, reagent vial 40 and, to a lesser extent, risertube 95.

Water flowing in a system, such as a water treatment and distributionsystem, comprises an unknown quantity of dosed stabilized hydrogenperoxide, which acts as a disinfectant. In one embodiment the dosedstabilized hydrogen peroxide is HUWA-SAN 25. The use of other stabilizedhydrogen peroxides is contemplated at various concentrations.

As shown in FIG. 2, there is provided apparatus 100 in accordance withone embodiment of the present invention. A close-up diagram of apparatus100 is provided in FIG. 3, which includes a further close-up diagram ofbuffer jar 10. Apparatus 100 diverts water from the system and intobuffer jar 10. Buffer jar 10 serves as a reservoir from where a watersample can be directed to a measuring cell 15 at a desired timeinterval. An overflow tube 50 returns water to the system, therebymaintaining a constant volume in the buffer jar 10 and a turn-over ofwater in the buffer jar 10. Water exiting buffer jar 10 may first passthrough a filter 55 to remove particulate matter. Optionally a filtermay be installed prior to water entering buffer jar 10 or in such otherlocations as to prevent or limit particulate matter from enteringapparatus 100. Sample water flow is directed from the buffer jar 10 tothe measurement cell under control of a supply valve 60, reagent isdirected to the measurement cell under control of a reagent valve 70,and a drain valve 80 operates to control fluid retention in, or drainingof, the measurement cell 15. Outflow from the measurement cell 15 isdirected to a waste drain 90. The measurement cell 15 has an upperopening and a lower opening 105. The lower opening is connected to anetwork of piping to allow filling and emptying of the measurement cell15 with sample water and reagent as required. The upper opening isconnected to a riser tube 95. The riser tube 95 extends upward to atleast the height of the water level in the buffer jar 10. The riser tube95 increases the efficiency of rinsing the measurement cell 15 byproviding added volume and force of the water movement. Plumbingconnects the elements to provide a conduit for fluid flow. For example,a first network of pipes connecting the source water to the buffer jar10, the buffer jar 10 to the supply valve 60, the supply valve 60 to themeasurement cell 15, and a second network of pipes connecting thereagent vial 40 to the reagent valve 70, the reagent valve 70 to themeasurement cell 15, the measurement cell 15 to the drain valve 80, andthe drain valve 80 to the waste drain 90. Such plumbing can be composedof PVC piping, or flexible tubing such as Tygon™ tubing, a combinationthereof, or other such conduit as desired.

The direction of flow of the various fluids is depicted in FIG. 5,showing (by arrows) the supply sample fluid flow through supply valve60, the supply reagent fluid flow from reagent valve 70 and the celldrainage from measurement cell 15 through drain valve 80.

In one embodiment, an apparatus 100 is provided that receives water froma system into a buffer jar 10, draws a first water sample from thebuffer jar into a measurement cell 15 to determine a null or backgroundreference measurement, removes the first sample, draws an aliquot ofreagent from a reagent vial 40 into the measurement cell 15 and draws asecond water sample from the buffer jar 10 into the measurement cell 15to determine a sample measurement. The null measurement is subtractedfrom the sample measurement and the difference value is interpretedrelative to a standard curve for a determination of hydrogen peroxideconcentration. A standard curve can be represented graphically or bymathematical expression of the curve. The mathematical expression isuseful in a digital system.

A side perspective view of FIG. 3 is provided in FIG. 4, showing lightemitter 25 and light receiver 30 on either side of measurement cell 15.The size and design of the measurement cell 15 will influence theaccuracy and efficiency of the measurement. Factors may include, but arenot limited to, the path length from a light emitter 25 to a lightreceiver 30, the light yield of the light source and the sensitivity ofthe light receiver 30, the measurement cell 15 composition and cell wallthickness, and distance between emitter 25 and receiver 30 elements. Thephysical parameters such as measurement cell wall thickness, path lengthand photodiode emitter and receiver equipment are fixed once chosen andtherefore can be compensated by hardware calibration and systemsettings. In a preferred embodiment, measurement cell 15 was custommilled from a single piece of thermoplastic polycarbonate, such asLexan™, with known techniques such as a CNC mill station.

In one embodiment, the measurement cell 15 has a width of 10 mm; cellwall thickness of 1 mm, and cell height is 19.5 mm. A measurement cellof these dimensions produces a sample volume of about 2 mL. Othervolumes are contemplated. These parameters were selected as optimaldimensions for this embodiment given a number of factors includingsufficient sample size for greatest accuracy and precision, as well ascost. The material used for the measurement cell was chosen based onlight transmission capabilities and resistance to degradation from waterand chemical reagents. All channels were polished to a high transparencylevel to maximize light transmission. Measurement cell 15 was polishedon the inside to enable maximum light transmission.

The measurement chamber shown in FIG. 6 comprises a light source 25 toemit light at a selected wavelength, measurement cell 15, and aphotodiode receiver 30. Preferably the light emitter 25 is a LED lightwhich emits light at 470 nm. The light is transmitted through the wallsof the measurement cell 15 containing the sample and the resultingnon-absorbed light is captured on photodiode 30. A small current isgenerated in the photodiode 30, which is measured by an operationalamplifier on the apparatus 100 and converted by an analog/digital (AD)convertor to an internal value of 1000, which is the resolution of themeasurement processor. This is the null value or zero reagent sample.

FIG. 9 shows a plot of absorption over concentration (mg/L) andestablishes that the light absorption of the reaction product is linearin relation to the concentration of hydrogen peroxide. The raw dataproduced by the photodiode and that of the AD convertor results in alogarithmic relationship to H₂O₂ concentration. With respect to unitsused to express concentration of H₂O₂ both mg/L and ppm are commonlyused. It is noted that 1 mg/L is equivalent to 1 ppm.

A reagent mixture was developed wherein a substrate reagent reacts withH₂O₂ to produce a reaction product in a stoichiometric relationship. Thereaction product is detected spectrophotometrically at a selectedwavelength and correlated to sample hydrogen peroxide concentration.Components of the reagent mixture do not interfere with the colorimetricmeasurement and are not effected by variable water characteristics suchas water hardness or trace metal or organic components. An effectiveamount of a surfactant is optionally added to improve reagent mixtureflow characteristics through the measurement cells and interconnectingchannels.

In one embodiment, the reagent mixture comprises the followingcompounds: potassium bis (oxalato) oxotitanate (IV) DI (Merck KGaA,Darmstadt, Germany); EDTA di-sodium salt dihydrate Titriplex III™ (MerckKGaA, Darmstadt, Germany); Surfactant: Polyoxyethylene (23) lauryl etherBrij™ 35 (30%) (Sigma-Aldrich); disolved in a solvent of sulfuric acid99%: p.a. 10% solution (Merck KGaA, Darmstadt, Germany).

The reagent mixture is prepared as follows for a 1,000 mL final volume,50 g potassium bis (oxalato) oxotitanate (substrate reagent), 0.2 g EDTAdi-sodium salt dehydrate, sulfuric acid 99% to a final dilution of 10%,and 1 ml of polyoxyethylene (23) lauryl ether.

Prior to use, the apparatus 100 is calibrated. Samples having knownconcentrations of hydrogen peroxide are measured and a standard curve iscreated by plotting the observed measurement cell output signalmeasurement against the known concentration. This curve can berepresented graphically (see FIG. 7) or by mathematical extrapolation.The concentration of hydrogen peroxide in an unknown sample is thendetermined with reference to the standard curve and the result reported,displayed or recorded either digitally, graphically or by otherconvenient means. Preferably, the standard curve includes a range ofknown samples spanning the range of concentrations to be measured, forexample from 0-150 mg/L (ppm). The standard curve comprising data pointsin the desired range (i.e. 0-150 mg/L) greatly increases the accuracy ofthe determination and is key to providing an accuracy of 0.1 mg/L.

A known concentration of hydrogen peroxide is required in order tocalibrate the apparatus. Perhydrol™ 30% for analysis EMSURE (TM) ACS,ISO from EMD Millipore Corporation of the Merck Group, was used. Othercertified trade solutions can also be used so long as the accuracy is inthe order of 99.999%. Standard solutions for spectrophotometricmeasurement are made between 0 to 100 mg/L which is the measurementrange of the apparatus of the present invention. The concentrations ofthe Perhydrol dilutions are confirmed and validated through a standardtitration method, for example iodometry. Subsequently a pre-determinednumber of measurements are carried out in the apparatus with these knownconcentrations and raw data (see Table 1) is generated to create astandard curve, as shown in FIG. 10.

TABLE 1 mg/L Raw Value Raw Value With Correction (*511/480) 0 480 511 5384 409 10 309 329 15 245 261 20 201 214 30 134 143 40 93 99 50 60 64 6042 45 70 31 33 80 24 26 90 20 21 100 12 13

The standard curve is used to calibrate apparatus 100. The value of rawvalue in Table 1 is the raw data of the measured sample at a scale of0-480 as determined by the photodiode. A value of 480 represents 0%absorbance (100% transmission) and represents a zero or null samplereflecting that there is no hydrogen peroxide in the sample. Theresolution of the Analogue-Digital (AD) convertor is 512 bit (0-511).Raw values are converted to digital values by multiplying by 511 anddividing by 480, a factor of 1.0646. The measured value is compensatedfor full scale. The standard curve is determined with the apparatus inone embodiment of the present invention, which measures in 10 bitresolution (1000 steps, 100 ppm/1,000=0.1 ppm resolution).

A standard curve is generated by using a number of data points. Thestandard curve becomes more accurate when more points are generated.Preferably, data points are biased in the lower range of detection, forexample 0-20 ppm and cover the entire range of desired detection, forexample 0-150 ppm. Once generated the standard curve can be representedmathematically for convenient use within an algorithm of the controlunit 35. Example 2 describes a standard curve and the mathematicalderivation of a H₂O₂ concentration based on the curve.

Calibration of the apparatus 100 to control for hardware variables isperformed for each measurement cell. Hardware variables include wallthickness of measurement cell, specific path length between lightemitter and photodiode light receiver, and the light yield of the lightemitter and the sensitivity of the photodiode light receiver. Prior touse a calibration adjustment to compensate for hardware variables isperformed. The measurement cell is filled with distilled water and themeasurement signal that originates from the photodiode is then measuredthrough an input potentiometer and adjusted to 1,000. This is theresolution of the measurement processor within the control unit 35. Whenreplacing the measurement cell, adjustment to the photodiode or LEDrecalibration is essential.

A pre-measurement software adjustment to the photodiode light receptor30 sets the photodiode 30 to a value of 1,000 based on a control watersample present in the measurement cell 15. A test water samplecontaining added reagent produces the colored complex and absorbs morelight in logarithmic dependence to the amount of hydrogen peroxide inthe sample (see FIG. 8). Consequently, the light that is not absorbedreaches photodiode receiver 30, resulting in a relatively smallercurrent in comparison with the reagent-free sample measurement. Ameasurement of the test sample containing hydrogen peroxide will belower than 1,000. The apparatus 100 measures the change in current. Byadjusting for the measurement in the reagent-free sample, any absorbancedue to water turbidity or composition of the water sample and chamberwalls is accounted for.

To achieve maximum accuracy and sensitivity during operation theapparatus 100 will run a control sample (test water alone) as areference point, a sample measurement is then made with sample waterplus reagent. By factoring in the control sample at each test sample,variability in water composition and turbidity are controlled for andaccuracy of the hydrogen peroxide concentration determination maximized.Preferably, an accuracy to 0.1 ppm is achieved.

In making a measurement, an aliquot of reagent is transferred from thereagent vial 40, and a water sample is transferred from buffer jar 10,into the measuring cell 15. The aliquot of reagent is kept small,however an excess of reagent compound can be provided such that in themeasuring cell 15, hydrogen peroxide is the limiting reactant in theassay mixture. For a reagent mixture prepared as described above and a1.5 mL sample volume, the volume of required reagent was 0.03 mL in oneembodiment. This is suitable to provide accurate detection of hydrogenperoxide to a maximum water sample concentration of 100 ppm.

It is noted that the volumes of sample and reagent are measuredprecisely and consistently in order to achieve precise measurements. Assuch, a control system comprising a software algorithm is used toprovide precise control of the valves in order to deliver the optimalamount of reagent to ensure accuracy. The valves are controlled by thecontrol unit 35 in a time-dependent manner. A valve time refers to thelength of time the valve is in the open position thereby allowing fluidflow. The valve time is correlated to a volume such that a known valvetime will result in the movement of a known volume. These times may varydepending on the specific characteristics of the valve in use and arereadily determined. In one embodiment, the reagent valve time has beenset to 30 milliseconds to allow a reagent volume of 0.03 mL to pass thevalve. Head pressure in the water or reagent system may affect thevolume of fluid that passes the valve during a given valve time.Compensation may be made in a variety of means. For example, the bufferjar 10 maintains a constant volume and therefore maintains a constanthead pressure, a similar reagent buffer jar may readily be incorporatedinto the system design. Alternatively, other designs may be incorporatedto provide consistent pressures such as the use of a pressurized headspace over the liquid. The control unit is programmed to provide thedesired valve time at each step of the measurement cycle. For examplevalve times for flushing and rinsing of measurement cell are differentthan the valve time to add the volume for a test water sample. Othervalve times are contemplated.

In one embodiment, the control unit 35 operates to measure hydrogenperoxide concentration in a water system by diverting samples toapparatus 100 on a periodic basis, such as every 2 minutes. Other timeintervals are contemplated. The steps comprise:

-   -   Measurement cell 15 is filled with sample water;    -   Measurement cell 15 is emptied, flushing measurement cell 15 to        keep it clean;    -   Measurement cell 15 is refilled with sample water;    -   Measurement is obtained and set as a zero measurement (null        value);    -   Measurement cell 15 is emptied;    -   Measurement cell 15 receives an aliquot of reagent from reagent        vial 40, and measurement cell 15 is filled with sample water;    -   Measurement is obtained as test measurement;    -   Empty measurement cell 15;    -   Fill and empty measurement cell 15 with sample water to rinse        cell;    -   Fill and keep measurement cell 15 full until the next        measurement is initiated.

The control unit includes analysis capability and determines the testsample hydrogen peroxide concentration by an algorithm that comprisesthe steps of: calculating the difference between null and test samplemeasurements; determining the concentration of hydrogen peroxide from astandard curve; reporting and/or recording the sample hydrogen peroxideconcentration.

In a further embodiment, the valve time (the time the reagent valve isopen) is variable and the control system calculates the required volumeof reagent. The objective is to optimize reagent use. The control systemcalculates the average H₂O₂ concentration of at least the two previoussamples and bases its next valve time on an expected H₂O₂ concentration.A limitation of this method results in inaccuracies when the hydrogenperoxide level fluctuates quickly, as the reagent dosing calculationlags the actual reagent requirement.

In an alternative embodiment, the control system is set to control valvetime, and thus reagent volume, based on multiple measurements of eachtest sample. The results are used by the control system to adjustreagent volume as required. In one embodiment for a measurement ofhydrogen peroxide in a test sample an aliquot of reagent is introducedinto the measurement cell, the measurement cell is half filled with testsample and a measurement conducted and recorded, the measurement cell isthen filled with test sample and a second measurement conducted. In oneembodiment, Measurement 2 is approximately half the reading ofMeasurement 1. If Measurement 2 is much less than half of Measurement 1,reagent is the limiting reactant and more reagent is required in thefollowing measurement sequence. If Measurement 2 is greater than half ofMeasurement 1, reagent is in excess and less reagent can be used in thefollowing measurement sequence. If Measurement 2 is approximately halfof Measurement 1, then the amount of reagent being used is optimal.

In an alternative embodiment, reagent use is optimized to conservereagent while still ensuring that an excess of reagent is providedrelative to the hydrogen peroxide concentration. At least two readingsare obtained from one sample and used to determine reagent requirement.The control system sets the reagent dose for the subsequent test samplebased on measurements obtained from a previous test sample. The stepsare as follows:

-   -   Measurement cell 15 is filled with sample water;    -   Measurement cell 15 is emptied, flushing measurement cell 15 to        keep it clean;    -   Measurement cell 15 is refilled with sample water;    -   Perform a zero measurement (null value);    -   Measurement cell 15 is emptied;    -   Measurement cell 15 is refilled but at the beginning of the fill        cycle, a timed addition of reagent from vial 40 is added to the        sample;    -   The measurement cell is filled halfway;    -   Measurement 1 is conducted;    -   The measurement cell is filled fully with sample water without        the addition of extra reagent;    -   Measurement 2 is conducted;    -   Comparison of Measurement 1 value and Measurement 2 value. This        step is necessary to determine if the amount of reagent is        sufficient in relation to the measured hydrogen peroxide value.        Measurement 2 is about half the reading of Measurement 1.    -   Empty measurement cell 15;    -   Rinse measurement cell 15 with sample water;    -   Fill and keep measurement cell 15 full until the next        measurement is required.

The complete sequence in one embodiment takes about 2 minutes. Thesmallest wait time between two sequences is about 10 seconds. Multipleapparatuses may be added in the same location with offset measurementtimes to provide continuous accurate measurements of hydrogen peroxideconcentrations in that area over time.

When developing the reagent for use with apparatus 100, considerationwas taken for various degrees of water hardness and the presence oftrace metals. The reagent can be used to measure the hydrogen peroxidecontent in water samples having a wide range of pH, temperature andwater hardness.

The control unit 35 may also include a dosing and control algorithm thatcompares the measured H₂O₂ concentration to a set point that defines thedesired concentration of H₂O₂ in the water treatment and distributionsystem from which the apparatus is diverting water to the apparatus 100for measurement. The set point may be defined as a discrete value suchas 8 ppm or as a range such as between 2-10 ppm. The control unit 35 isfurther configured to control a H₂O₂ dosing apparatus of the watertreatment and distribution system in response to the measured H₂O₂concentration and the set point. The dosing apparatus is locatedupstream of the apparatus 100 such that additions of H₂O₂ made by thedosing apparatus are monitored by the apparatus 100 and subsequentmeasurements are re-evaluated by the dosing algorithm relative to theset point. A water treatment and distribution system may have multipleapparatus 100 for measurement and multiple control units 35 distributedthroughout the system. Alternatively, one control unit 35 may obtaininput from multiple measurement apparatus 100 and control multipledosing apparatus. An exemplary dosing and control algorithm is aProportional-Integral-Derivative (PID) control algorithm. A PID controlis a common control algorithm used in industry and has been universallyaccepted in industrial control. PID controllers have robust performancein a wide range of operating conditions and their functional simplicityallows for ease of operation.

EXAMPLE 1

The objective of this example was to verify the accuracy of oneembodiment of the present invention, in the lowest range of 1-5 ppm bytesting samples having a known concentration of H₂O₂ in the range of 1-5ppm.

The sample was prepared as follows: A 350 mg/L standard solution ofhydrogen peroxide was prepared with 2 mL of 30% aqueous solution H₂O₂(Perhydrol (TM)), which was pipetted into a 2,000 ml volumetric flask.The solution was then topped with distilled water.

The exact concentration of this solution was determined analytically bymultiple iodometric titrations (see Table 2, below). The average resultof these titrations was taken to be the true concentration of theprepared solution of H₂O₂.

TABLE 2 Volume of sodium thiosulphate used in Correspondingconcentration titration (mL) of H₂O₂ (mg/L) Titration 1 10.45 355Titration 2 10.10 344 Titration 3 10.55 359

The final concentration of the H₂O₂ standard was calculated to be:

$\left\lbrack {H_{2}O_{2}} \right\rbrack = {\overset{\_}{X} = {\frac{355 + 344 + 359}{3} = {353\frac{mg}{l}}}}$

From the standard solution (350 mg/L H₂O₂) five dilutions were preparedwith a final concentration of 5, 4, 3, 2 and 1 mg/L H₂O₂. The quantitieswere pipetted from the H₂O₂ standard solution into 100 mL volumetricflasks to achieve these concentrations are shown below in Table 3.

TABLE 3 Target Volume pipetted concentration (mg/L) from standardsolution (ml) 5 1.42 4 1.13 3 0.85 2 0.57 1 0.283

The apparatus of one embodiment of the present invention was then usedto independently determine the concentrations in the above solutions ofH₂O₂. Three measurement runs were taken for each sample. The results ofthe apparatus measurements and average values are presented in Table 4.

TABLE 4 Predicted Measured Values of Average of measured conc. H₂O₂Apparatus (mg/L) results (mg/L) with (mg/L) Run 1 Run 2 Run 3 margin oferror 1.0 0.9 1.1 1.0 1.0 ± 0.1 2.0 2.0 2.1 2.0 2.0 ± 0.1 3.0 2.9 3.03.1 3.0 ± 0.1 4.0 4.0 3.9 4.0 4.0 ± 0.1 5.0 5.0 5.1 5.0 5.0 ± 0.1

The discrepancy between the predicted and measured values was assumed tobe solely due to inaccuracies in the apparatus' ability to measureconcentrations of H₂O₂. The values of the samples of H₂O₂ in solutionmeasured by the apparatus in one embodiment of the present invention arelinear. The apparatus itself is able to measure the concentration ofH₂O₂ in solution with a degree of accuracy of 0.1 mg/L in the range of1-5 mg/L H₂O₂.

EXAMPLE 2

A standard curve was generated and digitized data on the X-axis(vertical) plotted relative to hydrogen peroxide concentration on theY-axis (horizontal). Table 5 presents data for two known concentrations(Points A, B) of H₂O₂ and one unknown concentration (Point C).

TABLE 5 Point Measured value (X) Concentration (Y) (ppm) A 208 21.4 B 9242.23 C 142 To be determined

An equation to represent the line between two known points isdetermined. The known points are selected based on proximity to that ofthe unknown measurement such that the unknown lies between the two knownpoints. In this case, for the selected known data points, the differencein X values (delta X) and the difference in Y values (delta Y) iscalculated by:

Delta X=Xa−Xb=208−92=116

Delta Y=Ya−Yb=21.4−42.23=−20.83

Slope=dY/dX=−20.83/116=−0.18

Yc is the unknown H₂O₂ concentration represented by the coordinate Yc.Distance Xc:b from a known data point is calculated; Xc−Xb=142−92=50Yc=(Xc−Xb)*slope+Yb=(142−92)*−0.18+42.23=33.23

The unknown Yc can alternatively be calculated from point B (Xc−Xa)Yc=(Xc−Xa)*slope+Ya=(142−208)*−0.18+21.4=33.28

While the invention has been described with respect to a specificapparatus and method for measuring hydrogen peroxide in water, andcontrolling the level of hydrogen peroxide in water, it may equally beapplied to other apparatuses having various structures, so long as theyinclude the elements as described herein.

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 7. (canceled)
 8. (canceled)
 9. A method of for measuringhydrogen peroxide levels in a water treatment system using acolorimetric assay, the method comprising: a. transferring a first watersample to a measuring cell; b. determining a first absorbancemeasurement of light at a selected wavelength as a null measurement; c.removing the first sample from the measurement cell; d. transferring analiquot of a of a reagent compound capable of reacting with hydrogenperoxide to form a reaction product, the reaction product adapted toabsorb light at the selected wavelength proportional to the amount ofhydrogen peroxide in the water sample to the measurement cell; e.filling the measurement cell with a second water sample; f. determininga second absorbance measurement of light at the selected wavelength as atest measurement; g. emptying the measurement cell and rinsing withsample water; and h. determining the difference between the first andsecond measurements and comparing the difference against apre-determined standard curve of diluted hydrogen peroxide to determineand report the concentration of hydrogen peroxide in the water sample.10. The method of claim 9, wherein the reagent compound is potassium bis(oxalato) oxotitanate (IV) DI.
 11. The method of claim 10, wherein theselected wavelength is 470 nm.
 12. The method of claim 9, wherein thereagent comprises potassium bis (oxalato) oxotitanate (IV) DI, EDTAdi-sodium salt dihydrate and polyoxyethylene (23) lauryl ether mixed ina solvent.
 13. The method of claim 12, wherein the solvent is sulfuricacid 99%: p.a. 10% solution.
 14. The method of claim 9, wherein thepredetermined standard curve comprises data points from 0 ppm to 150ppm.
 15. The method of claim 9, wherein the predetermined standard curvecomprises data points from 0 ppm to 100 ppm.
 16. The method of claim 9,wherein the concentration of hydrogen peroxide in the water sample isdetermined to an accuracy of 0.1 mg/L
 17. The method of claim 9, whereinthe water treatment system comprises drinking water.
 18. A method formeasuring the concentration of hydrogen peroxide in a water treatmentsystem containing hydrogen peroxide on a periodic basis using acolorimetric assay, the method comprising: a. drawing a water samplefrom the water treatment system through a network of pipes into a bufferjar, the buffer jar comprising an overflow for maintaining a constantwater volume and constant head pressure in the buffer jar and formaintaining a turn-over of water in the buffer jar; b. controlling asupply valve in a time-dependent manner to cause a first water sample tobe received into a measurement cell from the buffer jar and obtaining afirst colorimetric measurement; c. controlling a supply valve in atime-dependent manner to cause a second water sample to be received intothe measurement cell from the buffer jar and reacting the hydrogenperoxide in the water sample with a reagent to form a reaction product,the reaction product adapted to absorb light from a light transmitter ata selected wavelength proportional to the amount of hydrogen peroxide inthe water sample; g. determining a difference between the first andsecond measurements, and h. comparing the difference against apre-determined standard curve of diluted hydrogen peroxide to determinethe concentration of hydrogen peroxide in the water sample.
 19. Themethod of claim 18, wherein the water sample comprises drinking water.20. The method of claim 18, wherein the method provides continuoussampling and determination of hydrogen peroxide concentration in thewater treatment system.
 21. The method of claim 18, wherein in theperiodic basis is between 10 seconds and 2 minutes.
 22. The method ofclaim 18, wherein the selected wavelength gives the maximum absorbanceof the reaction product.
 23. The method of claim 18, wherein thepredetermined standard curve comprises data points from 0 ppm to 150ppm.
 24. The method of claim 18, wherein the concentration of hydrogenperoxide in the water sample is determined to an accuracy of 0.1 mg/L25. The method of claim 18, wherein the reagent compound is potassiumbis (oxalato) oxotitanate (IV) DI.
 26. The method of claim 18, whereinthe selected wavelength is 470 nm.
 27. The method of claim 18, whereinthe reagent comprises potassium bis (oxalato) oxotitanate (IV) DI, EDTAdi-sodium salt dihydrate and polyoxyethylene (23) lauryl ether mixed ina solvent.